1. Field of the Invention
The present invention relates to optical polarization switches used to switchably transform the polarization state of light.
2. Description of the Related Art
Optical polarization switches are widely used. Commercially available optical polarization switches include Pockels cells and Kerr cells, which subject solid crystals or liquid media such as nitrobenzene to large applied voltages (&gt;1 kV) to obtain switching action via linear or quadratic electro-optic effects, respectively.
More common, and considerably more economical, are switches based on the electric field reorientation of polar molecules including those of the cyano-biphenyl group in liquid crystal (LC) cells such as the twisted nematic (TN) cell, the supertwisted nematic (STN) cell, and the electrically controlled birefringence (ECB) cell. These are ubiquitous in digital indicators and flat panel displays. The principles of operation and construction methods for such devices are described in Linear and Nonlinear Optics of Liquid Crystals, by S. T. Wu and I. C. Khoo (World Press, 1993).
Some switches incorporate integral polarizers, while other switches simply modulate the polarization state of light and the polarization modulation is then converted to an intensity change by means of external polarizer elements. In either case, the quality of switch action depends on the completeness of the modulation or polarization change; the degree to which it is achromatic, or works equally for all colors of light; the efficiency, or freedom from absorption or other loss; and the field-of-view, or range of angles over which the switch performs well. Other concerns include cost, ease-of-manufacture, and speed of response.
Fergason, in U.S. Pat. No. 4,385,806, teaches the use of two fixed retarders, each having retardance less than or equal to that of a liquid crystal element, located adjacent to the entrance polarizer of a TN liquid crystal cell, to increase the field-of-view. The two retarders are oriented with their slow axis parallel and perpendicular, respectively, to the polarizer; this, in turn, is parallel to the director of the LC device at the side adjacent to the polarizer. In another construction retarders are arranged on opposite faces of the cell, at 45.degree. relative to the director axis as it appears at each face. None of these elements are achromatic and they tend to exhibit only moderate field-of-view.
This same patent teaches the `surface mode` of operation for ECB cells, an enhancement which provides increased response speed by use of a thick cell, used over only a portion of its range. While the surface mode ECB cell offers increased speed, relative to a TN cell, the use of a thick liquid crystal layer compromises its field-of-view. Fergason teaches reducing the field-of-view loss by placing two retarders adjacent to the surface mode liquid crystal device. The result is that some of the diminished field-of-view is recovered but still limited and the switch action is not achromatic.
Gurther and Wechler, in U.S. Pat. No. 3,881,808, teach shutter systems employing two TN cells in series. The twist sense and rotational orientation of the two cells is unspecified. By using two cells, a rapid response is obtained for both the rise and fall time, as sought, e.g., for camera applications. However, unless the cells have an opposite sense of twist, and a particular orientation relative to one another, this switch multiplies the chromatic error of the individual cells and offers poor field-of-view for a similar reason. Even in the best case, only the off or undriven state is improved. When both cells are driven, significant leakage is exhibited in this switch for off-axis rays.
Wiener-Avnear, in U.S. Pat. No. 4,408,839, shows a TN device compensated for the polarization ellipticity of the exiting light by the addition of a second TN device with opposite twist sense oriented so that the exit director is orthogonal to the entrance director of the cell being compensated. This second cell is not driven electrically. Normally, a TN cell has significant chromatic error in the off state, but in the on or driven state is nearly achromatic. The Wiener-Avnear switch reverses this, in that the switch is nearly achromatic in the off state, and has an improved field-of-view, but suffers large chromatic errors and field-of-view limits in the on state. Also, this switch construction requires TN cells with both left- and right-handed twist which is undesirable in manufacture.
Kizaki et. al., in U.S. Pat. No. 5,126,868, teach a display comprising an STN cell with a twist helix angle of 160.degree. to 270.degree. arranged in series with a 90.degree. TN cell having an opposite sense of twist. Only the STN cell is electrically driven and the 90.degree. TN cell acts as a passive compensating layer to correct color errors developed in the STN cell. As a result, the usual compensating film employed with STN cells is omitted. Crossed polarizers are used with the entrance polarizer axis oriented at an angle of 35.degree. to 50.degree. to the director of the STN cell. The cell thickness d and the birefringence .delta.n are chosen so that d.multidot..delta.n for the compensating cell is less than d.multidot..delta.n for the electrically driven cell. Only modest field-of-view and efficiency are obtained by this switch.
Heynderickx et. al., in U.S. Pat. No. 5,221,978, teach two TN cells arranged in series with an integral polarizer therebetween. This provides rotationally symmetric iso-contrast curves which improves the useful field-of-view in some applications. Thin construction methods are taught to avoid ghost images. Although this construction offers improved contrast relative to the Gurther and Wechler switch, inherent losses are present. First, there is absorption of 10% or more due to the extra polarizer layer. Second, the radial performance is degraded since the technique involves orienting the two cells so that the low-transmission quadrant of one cell is superimposed upon the high-transmission quadrant of the other cell. This insures that, for any viewing angle, at least one cell exhibits poor transmission. As a result, the patented switch provides poor, but rotationally symmetric, off-axis response.
Funada, Kozaki, Matsura, and Wada describe a type of TN display with improved field-of-view in U.S. Pat. No. 4,436,379. This display comprises two LC devices in series with unspecified twist sense and relative orientation. Both devices are driven, and in the preferred embodiment, one has about 10% less retardance than the other. There is no clear teaching of any mechanism which might yield an improved field-of-view.
Sato and Shibuya, in U.S. Pat. No. 4,448,489, describe a display consisting of a dot-matrix element laminated with a segment element to achieve improved viewing quality relative to a dot-matrix display. Although the patent specifies values for total cell retardance in each element, the improvement is not related to birefringence cancellation of the type mentioned by Wiener-Avnear or Funada et. al. It does not provide an improved field-of-view per se, but does provide for cancellation of shadows and the displacement of reflected images as they pass through the thickness of the display.
Wiener-Avnear and Grinberg, in U.S. Pat. No. 4,466,702, disclose an undriven LC retarder used to compensate an ECB display. The compensating retarder is oriented with its slow axis perpendicular to the ECB device. The goal is to yield a net retardance of 0 for on-axis rays when the ECB device is undriven so as to obtain increased contrast. This patent does not address off-axis response, which is probably degraded.
Kaye teaches, in U.S. Pat. No. 4,497,542, two ECB devices arranged with the slow axes of each parallel to one another and the tilt angles in mirror symmetry about the cell normal to yield improved off-axis response, particularly for rays in the plane of the bent director when partially driven. The two cells are driven by the same voltage so as to best cancel one another for off-axis rays. While this construction is an improvement over simple ECB devices it still has a very limited field-of-view and, being a retarder element, is not an achromatic switch.
Bos et. al. teach the construction of a liquid crystal ECB cell, in U.S. Pat. No. 4,582,396, with opposed tilt sense at opposite faces. After a period of time with no voltage applied, the molecular axes relax to assume a non-operational twisted state with a twist of .pi. radians, for which reason it is termed a `pi` cell. This device exhibits, in a single cell, a similar optical result as the Kaye system which employs two cells. The cell has improved field-of-view because of the inherent mirror symmetry, and improved speed due to the pattern of hydrodynamic flow which eliminates back-flow and its attendant torques. Like many switches, the `on` rise time is quite rapid, and can be as short as 100 .mu.s. The relaxation time of such switch, although the fastest of any nematic switch device yet developed, is nonetheless 10 to 20 times longer at 1-2 ms. Finally, the optical switch action is based on simple retardance, and thus is not achromatic.
Buzak, in U.S. Pat. No. 4,583,825, describes how the field-of-view, which is generally worst at 45 degrees to the director of the LC at the entrance face, may be improved by placing a second cell and polarizer in series with the first, with the second cell rotated axially by 45 degrees. In this way, the contrast is always good for one of the two devices in series, and a high contrast is always obtained, although it appears to introduce a factor of 2 loss in efficiency in coupling between the two stages. It is thus similar to the Heynderickx switch, in that it redistributes the off-axis axial pattern at the cost of significantly lower throughput. This switch construction is of little use in a high-efficiency optical system.
Kalmanash and Fergason teach a multi-color display, in U.S. Pat. No. 4,770,500, which provides red, green, or yellow outputs. The switch comprises two ECB cells with their director orientations opposed in a push-pull configuration. This switch is used as an electro-optic switch capable of introducing plus or minus a quarter-wave. A quarter-wave plate is placed in series with this switch resulting in a retardance of 0 or a half-wave. This is used to select red or green, which have orthogonal polarizations coming from a pleochroic polarizer. The analyzer is a neutral polarizer. The speed of response is said to be improved because of the push-pull ECB device. These are discussed more fully in Fergason's U.S. Pat. Nos. 4,540,243 and 4,436,376. As a switch element, the push-pull configuration offers speed but is not achromatic. Further, the use of two elements in series produces twice as much perpendicularly aligned liquid crystal material as other switches which degrades the field-of-view when both cells are driven.
Use of film compensating layers with a single LC cell to improve the viewing angle for TN cells with a 90.degree. twist angle is taught by Haas in U.S. Pat. No. 5,375,006. Kikuchi et. al., in U.S. Pat. No. 5,440,413, teach the use of two biaxial plates to compensate a 90.degree. TN cell. Kanemoto et. al., in U.S. Pat. No. 5,380,459, teach the use of a polysiloxane polymer layer with a maximum refractive index perpendicular to the substrates of a cell in order to improve the viewing angle. Use of such compensating films is necessary in the construction of STN cells, having twist angles from 210.degree. to 270.degree., as described by Wada et. al. in U.S. Pat. No. 5,337,174. Akatsuka, in U.S. Pat. No. 5,523,867, teaches the use of a biaxial film to compensate a STN cell. Use of multiple retardation plates arranged on opposite sides of a STN cell is taught by Yoshimizu et. al. in U.S. Pat. No. 5,126,866. Ohnishi and Kishimoto, in U.S. Pat. No. 5,400,158, teach the use of films in conjunction with an optically anisotropic substance with a twisted structure to compensate a STN cell. Optical switches utilizing the ECB effect and including one or more layers adjacent one another between the polarizer and one side of the LC cell are described by Kanemoto et. al. in U.S. Pat. No. 5,175,638, which employs a film with its maximum optical index oriented normal to the cell faces. Bos, in U.S. Pat. No. 5,187,603, discloses a single film used to compensate a pi cell and, in U.S. Pat. No. 5,410,422, teaches the use of a negative birefringence material to compensate a pi cell. These prior art devices use one or more films to compensate a single liquid crystal cell with only moderate improvement in the overall viewing angle.
In summary, the aforementioned optical switches provide high efficiency, a wide viewing angle, or achromatic behavior, but none provide a switch which exhibits all of these properties using a single element. Those switches which use two LC cells do not incorporate means to reduce the effect of field-aligned molecules in the driven state, an effect which is twice as great in such switches. This materially degrades the off-axis performance thereby rendering the switch useless for high-performance applications. Conversely, those switches which use a single LC cell do not achieve achromatic action and a sufficiently wide field-of-view. Thus, no prior art optical polarization switch provides highly efficient, achromatic, angle-independent switch action as is required for field-sequential color filters, high-performance shutters, and similar applications.